Typically, as shown in FIG. 1, a wireless communication system 10 comprises elements such as client terminal or mobile station 12 and base stations 14. Other network devices which may be employed, such as a mobile switching center, are not shown. In some wireless communication systems there may be only one base station and many client terminals while in some other communication systems such as cellular wireless communication systems there are multiple base stations and a large number of client terminals communicating with each base station.
As illustrated, the communication path from the base station (BS) to the client terminal direction is referred to herein as the downlink (DL) and the communication path from the client terminal to the base station direction is referred to herein as the uplink (UL). In some wireless communication systems the client terminal or mobile station (MS) communicates with the BS in both DL and UL directions. For instance, this is the case in cellular telephone systems. In other wireless communication systems the client terminal communicates with the base stations in only one direction, usually the DL. This may occur in applications such as paging.
The base station with which the client terminal is communicating with is referred as the serving base station. In some wireless communication systems the serving base station is normally referred to as the serving cell. While in practice a cell may include one or more base stations, a distinction is not made between a base station and a cell, and such terms may be used interchangeably herein. The base stations that are in the vicinity of the serving base station are called neighbor cell base stations. Similarly, in some wireless communication systems a neighbor base station is normally referred to as a neighbor cell.
Duplexing refers to the ability to provide bidirectional communication in a system, i.e., from base station to client terminals (DL) and from client terminals to base station (UL). There are different methods for providing bidirectional communication. One commonly used duplexing method is the Frequency Division Duplexing (FDD). In FDD wireless communication systems, two different frequencies, one for DL and another for UL are used for communication. In a FDD wireless communication system, the client terminals may be receiving and transmitting simultaneously.
Another commonly used method is Time Division Duplexing (TDD). In TDD based wireless communication systems, the same exact frequency is used for communication in both DL and UL. In TDD wireless communication systems, the client terminals may be either receiving or transmitting but not both simultaneously. The use of the RF channel for DL and UL may alternate on periodic basis. For example, in every 5 ms time duration, during the first half, the RF channel may be used for DL and during the second half, the RF channel may be used for UL. In some communication systems the time duration for which the RF channel is used for DL and UL may be adjustable and may be changed dynamically. In some communication systems, a predefined set of configurations may be used to select between different DL and UL duration ratios as shown in FIG. 2. These predefined configurations are referred herein as TDD configurations.
Yet another commonly used duplexing method is Half-duplex FDD (H-FDD). In this method, different frequencies are used for DL and UL but the client terminals may not perform receive and transmit operations at the same time. Similar to TDD wireless communication systems, a client terminal using H-FDD method must periodically switch between DL and UL operation. All three duplexing methods are illustrated in FIG. 2.
The 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) wireless communication system is designed for low latency and high throughput applications. Supporting such applications requires the allocation of resources in a dynamic manner. This is different from the previous generation wireless communication systems which were designed for allocations that do not change for tens of seconds and even minutes or hours. In 3GPP LTE wireless communication system the resource allocation may change once every millisecond both in DL and UL.
The cost of such dynamic resource allocation is that the overhead for allocating resources is incurred every millisecond. To keep the overhead of resource allocation low while keeping the allocation dynamic, the 3GPP LTE wireless communication system employs several techniques. A control channel, called Physical Downlink Control Channel (PDCCH), is used for the purpose of dynamic resource allocation. The resource allocation message which is transmitted using the PDCCH is called Downlink Control Information (DCI). One of the requirements for a base station in a 3GPP LTE wireless communication system is the flexibility in addressing (sending resource allocation to) a particular client terminal through the PDCCH. This flexibility in turn requires the client terminal to search all possible PDCCH candidates within the control region of a subframe (SF) for possible resource allocation to it as shown in FIG. 3. This is referred to herein as blind PDCCH decoding and the portion of the control region in which the PDCCH search is performed is referred to as search space. The maximum number of decoding attempts in blind PDCCH decoding is 44 as specified in Release 8 and Release 9 of the specifications of 3GPP LTE wireless communication system. In later releases of specifications of 3GPP LTE-Advanced wireless communication system the number of candidates for blind PDCCH decoding may be increased even more. Furthermore, the increase in blind PDCCH decoding attempts for 3GPP LTE-Advanced wireless communication system may be proportional to the number of carriers supported for Carrier Aggregation (CA). In case a client terminal does not support CA, the number of decoding attempts in blind PDCCH decoding for a Release 10 or later client terminal may still be higher than or as high as in Release 8 and Release 9 3GPP specifications.
The information in PDCCH is protected by Forward Error Correction (FEC) coding as well as error detection. The error detection is based on a 16-bit Cyclic Redundancy Check (CRC). Different client terminals are identified in the 3GPP LTE wireless communication system using a type of identifier known as Radio Network Temporary Identifier (RNTI). Some RNTIs are of broadcast type which address more than one client terminal in a cell, whereas other RNTIs address a particular client terminal. In a PDCCH, a particular client terminal is addressed by the base station by scrambling the 16-bit CRC with the intended RNTI as shown in FIG. 4. The intended RNTI may be a broadcast RNTI or client terminal specific RNTI. The purpose of using the RNTI to scramble the CRC rather than including the RNTI in the payload is to reduce the overhead and to improve the performance of the FEC. The CRC encoded DCI message is then convolutionally encoded followed by interleaving and rate matching operations as shown in FIG. 5. In a 3GPP LTE wireless communication system, the convolutional code with constraint length K=7 is used as shown in FIG. 6. This means that the encoder may be in any one of the 2K-1=64 states. The rate matching may involve puncturing of some bits or repetition of some bits depending on the code rate for a given PDCCH.
In the client terminals during blind PDCCH decoding, the input to the PDCCH decoder may be from the signal transmitted by the serving base station or some random noise and interference signals from parts of the downlink signal where the serving base station may not be transmitting any information at all or may be transmitting information intended for other client terminals. In a given subframe only a few (typically two) out of all the blind PDCCH decoding attempts may have a useful signal transmitted by the serving base station intended for a particular client terminal. In case a client terminal decodes a PDCCH with passing CRC and the decoded RNTI matching with its own assigned RNTI when the base station is not actually transmitting a PDCCH for that client terminal, it is defined herein as invalid PDCCH decoding. The probability that a random 16-bit pattern matches the CRC for the payload portion of the data is 1/216. Considering that there are 44 blind PDCCH decoding attempts made by the client terminal per subframe (1 ms), the probability of getting an invalid PDCCH decoding per subframe is 44/216. Furthermore, the PDCCH CRC is checked in conjunction with multiple RNTIs that may be configured by the base station. A base station is referred to as an evolved Node B (eNB) in 3GPP LTE wireless communication system. For example, if on average two RNTIs are used by the client terminal at any given time, the probability of invalid PDCCH detection increases by a factor of two, i.e., (2*44)/216. This translates to about 0.00134 per millisecond (one subframe) or about 1.34 invalid PDCCH CRC pass per second. In case of a 3GPP LTE-Advanced wireless communication system with CA, this probability grows higher in proportion to the number of component carriers and the additional number of blind decoding attempts. It is to be understood that the invalid PDCCH detection may occur with higher probability if the number of blind PDCCH decoding attempts is increased in case LTE-Advanced and later releases of the 3GPP specifications.
In addition to invalid PDCCH detection, a duplicate PDCCH may be detected because of the different code rate used in FEC for different candidates of PDCCH.
The invalid PDCCH detection may lead to invalid DCI payload which in turn may lead to invalid resource allocation. The terms invalid PDCCH and invalid DCI are used interchangeably herein. Such invalid PDCCH detection can cause two types of problems. If the invalid PDCCH detection is related to DL resource allocation then it may cause the client terminal to receive the DL data that does not actually contain any information for that particular client terminal. This may result in increased power consumption in the client terminal. Furthermore, if there was a valid PDCCH transmitted for the client terminal in the same subframe, it may be missed since the client terminal may stop performing blind PDCCH decoding after successfully detecting the required number of PDCCHs. This may lead to reduced throughput for the client terminal and at the same time wasted resources (allocated but unused) and lead to reduced performance. If there is another downlink allocation using a broadcast RNTI in the same subframe, there may be a conflict in the resources allocated by the DCI message in the invalid PDCCH and the DCI message in the valid PDCCH for a broadcast RNTI. This may cause the client terminal to behave in unpredictable manner and could result in the client terminal not receiving the data intended for it.
For the UL direction, invalid PDCCH detection may result in the client terminal transmitting on resources that are not allocated to it. This may cause interference to one or more other client terminals which may be allocated those particular resources. This may lead to increased power consumption and reduced throughput for all the client terminals that may be transmitting on those particular allocated resources since the interference may lead to failed transmissions which may require retransmissions. Furthermore, if there was a valid PDCCH with UL resource allocation transmitted for the client terminal, it may be missed since the client terminal may stop performing blind PDCCH decoding after detecting the required number of PDCCHs. This may lead to reduced throughput for the client terminal and wasted resources (allocated but unused) in the UL.
The 3GPP LTE wireless communication system employs Hybrid Automatic Repeat Request (HARQ). Information regarding the HARQ protocol such as the process number, the Modulation and Coding Scheme (MCS), the Redundancy Version (RV), and whether a new transmission or retransmission may be taking place is sent as part of a DCI message. Invalid PDCCH detection may cause the HARQ Finite State Machine (FSM) running at the client terminal and at the eNB to be out of synchronization. For each DL resource allocation a corresponding HARQ acknowledgement must be sent in the UL. The exact allocation of UL resources for sending the acknowledgement is implicitly based on the exact resources corresponding to the PDCCH blind decoding candidate. The invalid PDCCH decoding then in turn leads to transmission of DL HARQ ACK/NACK (positive or negative acknowledgement) in the UL direction in the wrong UL resources and possibly interfering with other client terminals that may be sending their respective DL HARQ ACK/NACK in those resources.
When a client terminal is in spatial multiplexing mode, the DCI message may contain the allocation information such as the MCS, RV, and a new data indication for two codewords which may be mapped to the different layers of spatially multiplexed data transmission from the eNB. The transmission of HARQ ACK/NACK due to invalid DCI in those scenarios may cause further degradation because additional resources may be used for HARQ ACK/NACK transmission.
A client terminal may schedule decoding of the UL HARQ ACK/NACK for the UL transmission triggered by invalid PDCCH detection for UL resource. According to the 3GPP LTE wireless communication system HARQ protocol in UL, if a NACK is received in DL for a UL transmission, the client terminal is expected to automatically send a retransmission on the same resources as the original transmission causing further interference to other users. This process may continue until the maximum retransmissions are reached.
The invalid PDCCH decoding may lead to a series of problems that may compound both in DL and UL over a period of several radio frames.
Note that the invalid PDCCH detection rate mentioned earlier is only for one particular client terminal. A cell may typically serve a number of active users, in the order of dozens of client terminals. This means that the invalid PDCCH detection on a per subframe per cell basis can become very high and may disrupt the normal operation of the network.